PHP to check if Tickboxes are selected from HTML file

I would like to check from my PHP file, if checkboxes are selected on my Main.html page, when I click a button on this page. If so do some work in the PHP file and return results (JSON) on the HTML Page

I have the following code so far:

Below is the HTML code with the tickboxes and a script using AJAX to receive the response on the Main.html webpage (and as I will be doing some work within the PHP file on the server):

<form action="available_evidence">
       <div id ="checkbox1"><input type="checkbox" name="Fb_check"  id="Fb_check" value="Bike"> Facebook<br></div>
       <div id ="checkbox2"><input type="checkbox" name="Mess_check"  id="Mess_check" >Facebook Messenger<br></div>


        <div id ="errorMessage">
        <p>Error: No Evidence Avaialble.<br>Please Check Evidence Directory</p>
        </div>

    </form>

    <form id="display" method="post" action="PHP_Function_2.php">

        <input type="submit" class="learnButton" name="insert" value="Load Location Evidence" />

    </form>

        <script>
        $(function(){
          $("#display").submit(function(event) {
            event.preventDefault();

              $.ajax({
                     type: "POST",
                     url: $(this).attr('action'),
                     data: $(this).serialize(),
                     dataType: 'json',
                     success: function(data)
                     {                 
                        //display data...

                        $("#results").empty();


                        $("#results").append(data); 
                        console.log(data);

                        }
                   });

          });
        });

Below is the PHP_Function_2.php file in which I wish to check if the tickboxes are selected, if so do something, in this case just post that if either box is:

<?php

$test = "Fb Checked !";
$test2 = "Fb messenger checked";


//IF TICKBOX IS SELECTED

if (isset($_POST['Fb_check'])) {


echo json_encode($test);
// Checkbox is selected


} elseif(isset($_POST['Mess_check'])){

echo json_encode($test2);


}

elseif((isset($_POST['Mess_check'])) && (isset($_POST['Fb_check']))) 
{

echo json_encode($test);
echo json_encode($test2);

}
?>

I would really appreciate some help getting this to work, as I am new to PHP and have been stuck for a while. Thanks in advance.